1. Field of the Invention
This invention relates generally to a filter apparatus and, more particularly, to bandpass filters mounted on a printed circuit board for use in a communication system, such as a mobile telephone system.
2. Description of the Prior Art
In a duplex-type communication system, such as a mobile telephone system, a directional coupler or antenna coupling device is generally utilized to transmit and/or receive radio waves via a single antenna.
As shown in FIG. 1, when a transmission signal Su of an up-channel is generated by a transmitting circuit 1, the signal Su is supplied to an antenna 3 through a transmission bandpass filter 4 whose pass band covers signals of all up-channels. Then, the signal supplied to the antenna 3 is suitably transmitted to a base station (not shown). A down-channel signal Sd is generated by the base station and is received by antenna 3. The received signal Sd is supplied to a receiving circuit 2 through a reception bandpass filter 5 whose pass band covers signals of all down-channels.
In the mobile telephone system, the frequencies of the transmitted and received signals Su and Sd are in the high frequency range, for example 800 to 900 MHz, so as might be expected leakage of the signals frequently occurs.
To solve the above-described signal leakage problem, as shown in FIG. 2, the bandpass filters 4 and 5 are accommodated in a common shield casing 6. The output terminal of bandpass filter 4 and the input terminal of bandpass filter 5 are connected to coaxial connector 63 and the input terminal of bandpass filter 4 is connected to coaxial connector 64 and the output terminal of bandpass filter 5 is connected to coaxial connector 65. Input and output coaxial connectors 63, 64 and 65 are provided on different side surfaces of the shield casing 6. It is desired that an isolation characteristic exist between the terminals connected to coaxial connectors 63, 64, and 65, so that a response outside of the pass band is less than -60 dB.
Thus, a directional coupler is formed of bandpass filters 4 and 5 that are accommodated within a unitary shield casing 6. Nevertheless, this directional coupler has an unavoidable defect in that the directional coupler formed as a single body requires three separate coaxial connectors 63, 64, and 65. Thus, the antenna apparatus is made expensive. Further, it is difficult to mount the antenna apparatus on a printed circuit board because the three coaxial connectors 63, 64, and 65 are located on different side surfaces of shield casing 6 as shown in FIG. 2.